TIN(IV) CHLORIDE–ZINC CHLORIDE

1

Tin(IV) Chloride–Zinc Chloride

1

SnCl

4

–ZnCl

2

(SnCl

4

)

[7646-78-8]

Cl

4

Sn

(MW 260.51)

InChI = 1/4ClH.Sn/h4*1H;/q;;;;+4/p-4/f4Cl.Sn/h4*1h;/q4*-1;m
InChIKey = HPGGPRDJHPYFRM-LUCXXLOKCB
(ZnCl

2

)

[7646-85-7]

Cl

2

Zn

(MW 136.29)

InChI = 1/2ClH.Zn/h2*1H;/q;;+2/p-2/f2Cl.Zn/h2*1h;/q2*-1;m
InChIKey = JIAARYAFYJHUJI-ATOKYBHQCC

(catalyst for bond formation between silyl nucleophiles and

carbonyl groups or enones

1

)

Physical Data:

SnCl

4

: mp −33

◦

C; bp 114.1

◦

C; d 2.226 g cm

−

3

;

ZnCl

2

: mp ca. 290

◦

C; bp 732

◦

C.

Solubility:

both reagents react violently with water. SnCl

4

: sol

cold H

2

O; dec hot H

2

O; sol alcohol, ether, CCl

4

, benzene,

toluene, acetone. ZnCl

2

: sol H

2

O (432 g/100 g at 25

◦

C, 614

g/100 g at 100

◦

C); 2% HCl (4 g/1 mL); alcohol (1 g/1.3 mL);

glycerol (1 g/2 mL); freely sol acetone.

Form Supplied in:

blend not commercially available. Anhydrous

SnCl

4

: colorless liquid; 1 M soln CH

2

Cl

2

or heptane; widely

available. Anhydrous ZnCl

2

: white, odorless granules, lumps,

or rods; 1 M soln in Et

2

O, 0.5 M soln in THF; widely available.

Purification:

SnCl

4

: heat to reflux with mercury or P

2

O

5

for sev-

eral hours, then distil under reduced nitrogen pressure into re-
ceiver with P

2

O

5

. Redistil. Typical impurities: hydrates. ZnCl

2

:

heat to reflux in dioxane (100 g/800 mL) with zinc dust (10 g),
filter hot, and cool to precipitate ZnCl

2

. Crystallize from diox-

ane. Impurities: H

2

O, zinc oxychloride.

Handling, Storage, and Precautions:

both reagents are hygro-

scopic and should be stored in a glove box or over P

2

O

5

to

minimize exposure to moisture. Containers should be flushed
with N

2

or Ar and tightly sealed. Perform all manipulations un-

der N

2

or Ar. Solvating SnCl

4

with H

2

O is highly exothermic.

Use in a fume hood.

Introduction. The Tin(IV) Chloride–Zinc Chloride blend

1

is one of many Lewis acid blends, such as Sn(OTf)

2

–Bu

3

SnF,

2

SnCl

4

–Sn(OTf)

2

,

3

SbCl

5

–Sn(OTf)

2

,

4

TMSCl–SnCl

2

,

5

TrCl–SnCl

2

(Tr = trityl),

6

SnO–TMSOTf,

7

and GaCl

3

–AgClO

4

,

8

which are effective catalysts in carbon–carbon bond forming
reactions. The active catalyst is believed to be

+

ZnCl–SnCl

5

−

which is formed prior to the addition of organic reactants. Single
Lewis acids (SnCl

4

, TiCl

4

, etc.) promote these reactions, but do

not catalyze them.

9

Ethynylation of Acetals and Aldehydes.

1

The SnCl

4

–ZnCl

2

blend is the most useful catalyst (10 mol %) for the preparation of
secondary propargylic ethers from 1-trimethylsilyl-1-alkynes and
acetals (eq 1).

1

Conventional promoters such as TrCl–SnCl

2

and

TMSCl–SnCl

2

are not effective, and Sn

II

–Sn

IV

, Sn

II

–Ti

IV

, and

Zn

II

–Ti

IV

blends provide lower yields. Moderate yields (29–53%)

are obtained for acetals with large alkoxy groups (R

2

); however,

cyclic acetals, such as 1,3-dioxolane, do not react. Aromatic and

conjugated dimethyl acetals provide dipropargyl derivatives as
side products. Ethynylation of aldehydes is accomplished by form-
ing intermediate hemiacetal-like compounds from aldehydes and
alkoxytrimethylsilanes in the presence of the Lewis acid blend.
These intermediates then undergo reaction with 1-trimethylsilyl-
1-alkynes to form the desired secondary propargylic ethers (eq 2).

R

1

OR

2

OR

2

R

3

TMS

R

1

R

2

O

R

3

+

10% SnCl

4

–ZnCl

2

CH

2

Cl

2

, rt

(1)

(1)

13–85%

R

1

R

2

O

R

3

1. cat. SnCl

4

–ZnCl

2

2. R

2

OTMS

3. (1)

(2)

R

1

H

O

0–78%

Allylation

of

Secondary

Propargylic

Ethers

and

Aldehydes.

1

Propargylic ethers are allylated by Allyltrimethyl-

silane in the presence of the SnCl

4

–ZnCl

2

blend (eq 3). Thus

acetals can be transformed to 1,5-enynes in one pot with
sequential nucleophilic additions. The blend also catalyzes
the allylation of aldehydes by allyltrimethylsilane, yielding
homoallylic alcohols in good yields (61–74%).

MeO

Ph

cat. SnCl

4

–ZnCl

2

(3)

Ph

TMS

Ph

Ph

56%

Aldol and Michael Reactions.

1

The SnCl

4

–ZnCl

2

blend is

an effective catalyst in the aldol reaction of silyl enol ethers with
aldehydes (eq 4), acetals (eq 5), or ketones. The product anti/syn
ratios are variable (32:69 to 89:11). The blend also catalyzes the
Michael addition of silyl enol ethers with α,β-unsaturated ketones
(eq 6), yielding alkylation products (84–100%) with anti selectiv-
ity (anti/syn = 55:45 to 87:23)

SnCl

4

–ZnCl

2

Ph

OTMS

Ph

H

O

Ph

Ph

O

OH

+

(4)

99%

SnCl

4

–ZnCl

2

Ph

OTMS

Ph

OMe

OMe

Ph

Ph

O

OMe

+

(5)

99%

+

(6)

Ph

OTMS

Ph

O

SnCl

4

–ZnCl

2

O

O

87%

Related Reagents. Tin(IV) Chloride; Zinc Chloride.

1.

(a) Hayashi, M.; Inubushi, A.; Mukaiyama, T., Bull. Chem. Soc. Jpn. 1988,
61

, 4037. (b) Hayashi, M.; Inubushi, A.; Mukaiyama, T., Chem. Lett. 1987,

1975.

Avoid Skin Contact with All Reagents

2

TIN(IV) CHLORIDE–ZINC CHLORIDE

2.

(a) Mukaiyama, T.; Uchiro, H.; Kobayashi, S., Chem. Lett. 1989, 1001.
(b) Kobayashi, S.; Mukaiyama, T., Chem. Lett. 1989, 297. (c) Kobayashi,
S.; Uchiro, H.; Fujishita, Y.; Shiina, I.; Mukaiyama, T., J. Am. Chem. Soc.
1991, 113, 4247.

3.

Mukaiyama, T.; Shimpuku, T.; Takashima, T.; Kobayashi, S., Chem. Lett.
1989, 145.

4.

Kobayashi, S.; Tamura, M.; Mukaiyama, T., Chem. Lett. 1988, 91.

5.

(a) Iwasawa, N.; Mukaiyama, T., Chem. Lett. 1987, 463. (b) Mukaiyama,
T.; Wariishi, K.; Saito, Y.; Hayashi, M.; Kobayashi, S., Chem. Lett. 1988,
1101.

6.

(a) Mukaiyama, T.; Kobayashi, S.; Tamura, M.; Sagawa, Y., Chem. Lett.
1987, 491. (b) Mukaiyama, T.; Sugumi, H.; Uchiro, H.; Kobayashi, S.,
Chem. Lett. 1988

, 1291.

7.

Mukaiyama, T.; Uchiro, H.; Kobayashi, S., Chem. Lett. 1990, 1147.

8.

Mukaiyama, T.; Ohno, T.; Nishimura, T.; Suda, S.; Kobayashi, S., Chem.
Lett. 1991

, 1059.

9.

(a) Mukaiyama, T.; Narasaka, K.; Banno, K., Chem. Lett. 1973, 1011. (b)
Mukaiyama, T., Angew. Chem., Int. Ed. Engl. 1977, 16, 817. (c) Narasaka,
K.; Soai, K.; Aikawa, Y.; Mukaiyama, T., Bull. Chem. Soc. Jpn. 1976, 49,
779.

Stephen Castellino

Rhône-Poulenc Ag. Co., Research Triangle Park, NC, USA

David E. Volk

North Dakota State University, Fargo, ND, USA

A list of General Abbreviations appears on the front Endpapers